1. Field of the Invention
This invention relates to a zoom lens barrel, and more particularly to a zoom lens barrel in which zooming and focusing can be effected by a single drive source.
2. Description of the Related Art
In a camera with a zoom lens, generally zooming is effected by moving lens groups in the direction of the optical axis by a cam and the like while focusing is effected by moving the lens groups in a manner different from that in zooming.
As the method of moving the lens groups of the zoom lens in such a manner, there have been known the following methods.
(1) Separate drive sources are used for zooming and focusing.
(2) Drive force of a single drive source is divided and transmitted to the zooming mechanism and the focusing mechanism.
(3) A single drive force is used and zooming and focusing are alternately effected on a continuous locus. See Japanese Patent Publication No. 6(1994)-100707.
(4) A cam ring rotated by a single drive source is provided with first and second cam grooves which respectively move first and second groups in the direction of the optical axis in response to rotation of the cam ring. When the cam ring is rotated in one direction (toward telephoto side or wide-angle-side), the first and second lens groups are moved back and forth by way of the first and second cam grooves whereby the lens is zoomed. When the cam ring is rotated in the other direction after the zooming action, the second cam groove does not act on the second lens groove for a predetermined focusing segment and only the first lens group is moved, whereby the lens is focused. See Japanese Unexamined Patent Publication No. 9(1997)-49958.
However the method of (1) is disadvantageous in that since two drive sources are required, cost of the camera is increased. The method of (2) increases the number of components, which results in a larger size of the camera. The method of (3) is disadvantageous in that zooming cannot be effected continuously and focusing can be effected only at a plurality of particular focal lengths (a step zoom).
Further in the method of (4), though continuous zooming is possible, the following problem arises. That is, the second cam groove is provided with a first cam surface which acts when zooming from the wide-angle-side to telephoto side and a second cam surface which acts when zooming from the telephoto side to the wide-angle-side and the first and second cam surfaces are opposed to each other wide spaced in the direction of the optical axis from a cam follower which is provided on a lens holder for the second lens group and is in engagement with the second cam groove. That is, there is play in the direction of rotation of the cam ring between the cam groove and the cam follower. Accordingly, an unintentional movement of the second lens group can occur under its gravity of the like during zooming, which can result in undesirable zooming, unless the cam follower is pressed against the first or second cam surface by an urging means such as a spring. However when such an urging means is provided, the drive system must operate overcoming the force of the urging means during focusing, which adds to the load on the drive source.